This research is directed towards examining the molecular signals which a malignant stem cell, embryonal carcinoma (EC), recognizes under two conditions: (1)\its continued growth as a stem cell; and (2)\its differentiation to nonmalignant endodermal cells. The probes for investigating the molecular signals are a unique class of polyoma virus mutants which have been isolated in this laboratory. The mutants overcome the wildtype polyoma block in productive infection of EC cells and some of the mutants have only a single base pair change difference from the wildtype virus. We are using small restriction fragments from the mutant and wildtype polyoma viruses in recombinant DNA constructions with other nonpolyoma genes. These recombinant DNA constructions are being introduced into the EC cells and differentiated derivatives. Our findings suggest that mutant sequences enhance the expression of nonpolyoma genes in EC cells. Our current studies involve: (1)\an examination of the nature of this gene expression enhancement; (2)\an examination of the sequence requirements for enhancement of differentiated cells derived from EC cells; and (3)\the search for cellular sequences which might have a similar function.